Single crystal spot patterns

The organic matrix follows the same pattern. By means of electron diffraction analysis it can be shown that the protein matrix is and remains crystalline and that the structural order of the diffraction pattern is that of a microcrystalline film of gold (Fig. 23)224). It is remarkable that the matrix proteins in other species also produce a single crystal spot pattern. This implies that isolated pieces of organic matrix which are in the order of a few microns thick represent single crystals. Whatever the term single crystal" may mean to the various researchers43, in the context... 

The existence of a superstructure was revealed by satellite spots in the XRD single crystal diffraction pattern of partly dehydrated goethite. The superstructure was considered to be an in-... 

Looking at the microstructure for samples above and below the maximum in coercivity, Fig. 21 shows that the FePt islands become interconnected above the coercivity maximum while below the maximum the islands are well separated. In the insets of Fig. 21(a) and (b) are the selected area diffraction (SAD) patterns for the samples. These indicate a single crystal FCT pattern with (001) orientation. Adjacent to the FePt diffraction spots are the (001) MgO single crystal spots indicating a slight mismatch in the lattice spacing of the two materials and a good epitaxial relationship between the two. 

Crystal geometry Moving crystal-spot pattern Single crystal... 

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

Figure 16-17. Left transmission electron micrograph of small single crystals of Ooct-OPV5 scale bar 5 pnt. The arrows indicate the 6-axis direction. Right electron diffraction pattern of the same single crystals. The arrow indicates the 613 relteclion spot (crysial dimensions 5x40 pm2 Philips STiiM CM 12 operated at 120 kV. lnslilul Charles Sudron, Strasbourg).

As illustrated by Eig. 4.13, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample. This technique is referred to as energy-dispersive X-ray analysis (EDX). 

Figure 1. Electron diffraction pattern examples a - spot pattern from a single crystal, b -oblique texture pattern tilted by 60°, c - powder ring pattern.

If a single crystal is rotated in a monochromatic X-ray beam, a pattern of spots of reinforced X-rays can be recorded, traditionally on a photographic film placed behind the crystal perpendicular to the primary beam (giving the so-called Laue photographs). Nowadays, X-ray diffractometers use electronic photon counters as detectors. Since, as noted above, different atoms have different X-ray scattering powers, both the positions and... 

Generally, LEED experiments are conducted on specified faces of single crystals. When this is done, the diffraction pattern produced consists of a series of spots with a location, shape, and intensity that can be interpreted in terms of the surface structure. We focus attention on what can be learned from the location and shape of the spots since the study of intensity is beyond the scope of this book. It is generally assumed that the surface examined by LEED is an extension of an already-known bulk crystal structure. The correctness of this assumption can be tested, and results are often expressed in terms of modifications of the three-dimensional structure at the surface. Before we turn to the LEED patterns below, we must first figure out how they are read. 

When an X-ray beam passes through such a fibre perpendicular to its length, the pattern produced is of the same type as that given by a single crystal rotated about a principal axis. All orientations perpendicular to the fibre axis are already present in the specimen, so that the effect of rotation is produced. Examples are shown in Plate X. The reflections are less sharp than those produced by single crystals, for two reasons firstly, the orientation of the crystals in the fibre is not perfect, so that each spot is drawn out to the form of a short arc, and secondly, in most polymer fibres the crystals are so small that the reflections are inevitably more diffuse than those of la rge crystals (see p. 437). 

In fibres of some polymers, made under certain conditions, the crystalline regions are found to be tilted with respect to the fibre axis in a well-defined crystallographic direction. This is a very valuable feature, because the diffraction patterns of specimens in which this type of orientation occurs are of precisely the same form as tilted crystal diffraction patterns of single crystals rotated round a direction inclined to a principal axis. The unit cell cannot be obtained directly, for 90° oscillation tilted crystal photographs are required for direct interpretation, but unit cells obtained by trial can be checked by the displacements of diffraction spots from the layer lines this is a severe check, and consistent displacements would leave no doubt of the correctness of a unit cell. This procedure played an effective part in the determination of the unit cell of polyethylene terephthalate (Daubeny, Bunn, and Brown, 1954). 

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